1. Field of the Invention
The invention generally relates to a power converting controller and a light-emitting diode driving circuit, and more particularly, to a power converting controller and a light-emitting diode driving circuit with open-circuit protection function.
2. Description of Related Art
In comparison with a traditional illumination light source, light-emitting diodes (LEDs) have advantages of high light-emitting efficiency, low power consumption and long lifetime, and the production cost thereof is being steadily reduced, so that people more and more broadly adopt the LEDs. An LED driving circuit usually is an isolated or a non-isolated constant-current power to drive a light-emitting diode string (LED string) and controls the current flowing through the LEDs through detecting current.
FIG. 1 is a schematic circuit diagram of a traditional LED driving circuit. Referring to FIG. 1, an LED driving circuit includes a controller 10, a converting circuit 35, a power feedback circuit 40 and an LED string 30. In the example case, the converting circuit 35 is a buck converting circuit, which includes an inductor L, a capacitor C, a rectifying diode D and a switch SW. The converting circuit 35 is coupled to an input power source VIN at the input terminal thereof and to the LED string 30 at the output terminal thereof so as to drive the LED string 30 for emitting light. A current-detecting resistor R is coupled to the LED string 30 through the switch SW and generates a current-detecting signal IFB according to the current flowing through the LED string 30. The current-detecting terminal CS of the controller 10 receives the current-detecting signal IFB and thereby generates a control signal ‘Gate’ at the driving terminal DRV thereof for switching the switch SW of the converting circuit 35 and further controlling the converting circuit 35 to convert the power. In this way, the LED string 30 can stably emit light. The converting circuit 35 charges an initial capacitor Cin through the power feedback circuit 40 as switching the switch SW. The initial capacitor Cin is coupled to the driving voltage terminal VDD of the controller 10 so as to provide the power to the controller 10.
In a traditional LED driving circuit, such as the one of FIG. 1, the circuit does not specifically detect whether or not the LED string is open-circuit, instead, employs the under-voltage lock out protection (UVLO protection) of the controller 10 to achieve the open-circuit protection function.
In more details, if any LED in the LED string 30 is damaged, it would causes open-circuit. When the LEDs occur open-circuit, even the switch SW is turned on, the current is unable to flow through the inductor L via the LED string 30, and it is unable to store the energy in the inductor L as well, which results in gradually reducing the stored energy (voltage) of the initial capacitor Cin. When the voltage is reduced to a voltage value of UVLO protection of the controller 10, the controller 10 is triggered to execute the UVLO protection so that the controller enters a latch status.
In the practical applications however, sometimes, the input power source VIN may be unstable for a while, in particular, when the input power source VIN is formed by rectifying the commercial power. When the unstable input power source VIN causes the voltage of LED string 30 lower than the minimum turn-on voltage, the LED string 30 fails conducting the current, this situation is equivalent to the above-mentioned situation where the LED string 30 occurs open-circuit, which would trigger the UVLO protection of controller 10 and enter a latch status. The controller 10 will remain in a latch status even when the input power source VIN resumes to the stable situation, which makes the LED string 30 abnormally stop lighting.